Several hundred for plugs? I must have a cheaper mechanic than I thought. Agree that they need to be changed as does tranny fluid and fuel filter. Ford claims in the owner's manual that the differentials are considered lubricated for life and do not need to be checked or changed unless aleak is suspected, service is required, or the axle assembly has been submerged in water. I would at the very least have it checked before outright changing it. Agree with the coolant too.

Good to know on that one. I am going to hold off even though I am getting close to it till I make a decision about new gears. Would be a waste of $$ to get it changed just to do it a gain a few months down the road.